When cells are faced with physiological or pathological stresses, they respond by adapting in any of several ways, one of which is metaplasia. It is a benign (i.e. non-cancerous) change that occurs as a response to change of milieu (physiological metaplasia) or chronic physical or chemical irritation (pathological metaplasia). One example of pathological irritation is cigarette smoke that causes the mucus-secreting ciliated pseudostratified columnar respiratory epithelial cells that line the airways to be replaced by stratified squamous epithelium, or a stone in the bile duct that causes the replacement of the secretory columnar epithelium with stratified squamous epithelium (Squamous metaplasia). Thus, metaplasia refers to the change or replacement of one type of epithelium by another. More specifically, metaplasia is an adaptation that replaces one type of epithelium with another that is more likely to be able to withstand the stresses it is faced with. It is also accompanied by a loss of endothelial function, and in some instances considered undesirable. This undesirability is underscored by the propensity for metaplastic regions to eventually turn cancerous if the irritant is not eliminated.
The medical significance of metaplasia is that in some sites where pathological irritation is present cells may progress from metaplasia, to develop dysplasia, and then malignant neoplasia (cancer). Thus, at sites where abnormal metaplasia is detected, efforts are made to remove the causative irritant, thereby decreasing the risk of progression to malignancy.
In the field of cervical image analysis, a transformation zone (TZ) is an area in the uterine cervix, where columnar epithelium is replaced by squamous epithelium. This is the region where the cancer occurs in the uterine cervix. During a colposcopy procedure, a 3-5% acetic acid solution is applied to the cervix. Acetic acid causes cellular dehydration and reversible coagulation of intracellular proteins, thus reducing the transparency of the epithelium. This results in temporary whiteness of the epithelium, i.e., aceto-white epithelium.
The speed with which this whiteness appears and disappears depends on the number of cells, amount of cytoplasm and nucleus size. Unfortunately, not all the areas of aceto-white epithelium indicate the presence of premalignant disease, for example areas of metaplastic epithelium are also aceto-white.
Following application of acetic acid, 3-5% of Lugol's iodine stain is used. Lugol's iodine, also known as Lugol's solution is a solution of elemental iodine and potassium iodide in water, named after the French physician J. G. A. Lugol. Lugol's iodine solution is often used as an antiseptic and disinfectant, for emergency disinfection of drinking water, and as a reagent for starch detection in routine laboratory and medical tests. These uses are possible since the solution is a source of effectively free elemental iodine, which is readily generated from the equilibration between elemental iodine molecules and triiodide ions in the solution. Normal squamous epithelium is rich in glycogen and stains dark brown with iodine (iodine positive), whereas premalignant squamous epithelium and endo-cervical epithelium is deficient in glycogen and does not stain to iodine (iodine negative). However, the mature metaplastic epithelium in most women is glycogen rich and stains dark brown to Lugol's iodine.
Aceto-whiteness detection, representation and quantification are important features of any automated cervical image analysis systems. However, not only the premalignant lesions but also the immature metaplasia shows aceto-white characteristics. This causes undesirable false alarms for aceto-white regions in automated colposcopic image analysis systems.